Id just fill it up more often mate - the water bottle is in a complete pain in the arse place behind the front wing - Its definitely Bumper off, probably the n/s/f wheel and youll have to get underneath too!
I just make it a routine - Big bottle of warm water in the morning, chuck some over the windows to clean them and get any ice off (of not it stops them steaming up anyway!) Some over the jet dispersers, and stick the rest in your reservoir!